Highly stable amorphous silicon thin film transistors and integration approaches for reliable organic light emitting diode displays on clear plastic

Hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) are currently in widespread production for integration with liquid crystals as driver devices. Liquid crystal displays are driven in AC with very low duty cycles and therefore fairly insensitive to the TFT threshold voltage rise which is well-known in a-Si:H devices. Organic light-emitting diodes (OLEDs) are a future technology choice for flexible displays with several advantages over liquid crystals. In contrast to liquid crystal displays, however, OLEDs are driven in DC and thus far more demanding in terms of the TFT stability requirements. Therefore the conventional thinking has been that a-Si:H TFTs are too unstable for driving OLEDs and the more expensive poly-Si or alternative TFT technologies are required.;This thesis defies the conventional thinking by demonstrating that the knowledge of the degradation mechanisms in a-Si:H TFTs may be used to enhance the drive current half-life of a-Si:H TFTs from lower than a month to over 1000 years by modifying the growth conditions of the channel and the gate dielectric. Such high lifetimes suggest that the improved a-Si:H TFTs may qualify for driving OLEDs in commercial products. Taking advantage of industry-standard growth techniques, the improved a-Si:H TFTs offer a low barrier for industry insertion, in stark contrast with alternative technologies which require new infrastructure development. Further support for the practical advantages of a-Si:H TFTs for driving OLEDs is provided by a universal lifetime comparison framework proposed in this work, showing that the lifetime of the improved a-Si:H TFTs is well above those of other TFT technologies reported in the literature.;Manufacturing of electronic devices on flexible plastic substrates is highly desirable for reducing the weight of the finished products as well as increasing their ruggedness. In addition, the flexibility of the substrate allows manufacturing bendable, foldable or rollable electronic systems which is not possible with conventional rigid substrates. The most reliable TFTs require a temperature higher than that possible with existing clear flexible plastic substrates. Successful integration of a-Si:H TFTs with OLEDs on new high temperature flexible clear plastic substrates, capable of being processed at 300°C, is presented in this thesis. Controlling the mechanical stress and adhesion of the layers is found to be critical at high process temperatures to avoid cracking and delamination on clear plastic, and TFTs with a lifetime of 100 years on clear plastic have been achieved. In addition, a new "inverted" integration technique is demonstrated both on glass and clear plastic to allow the programming of standard bottom-emission OLEDs with a-Si:H TFTs independent of the OLED characteristics which may change over time and vary from device to device in manufacturing. This technique also enhances the pixel drive current by nearly an order of magnitude for the same programming voltage. Finally, an approach for the design of reliable pixels is presented. Based on the individual TFT and OLED device stability, a guideline to the overall circuit configuration that will provide the most stable light emission is provided.